Conventional video projectors are generally relatively large, heavy, and sensitive, so that they are less suitable for mobile use. In addition, such devices have a comparatively high power consumption and the colors produced sometimes appear faint, for example, if sunlight is incident on the projection surface.
The technologies which may be used in miniaturized projectors include laser scanners. In laser scanners, laser beams emitted by laser diodes are moved in such a way that they scan a projection surface. Laser beams are generally moved along horizontal scanning lines. When the entire horizontal width of the desired projection surface has been traveled through along a scanning line, the scanning line is vertically shifted.
A color image is generated by superimposing laser beams of different colors and by targeted projection and non-projection of the laser beams. This takes place by using such a high repetition frequency that an overall image arises for the human eye, similarly as in the case of scanning of the luminescent coating of a CRT television using an electron beam. Such laser scanners are small and generate images having strong colors. In addition, they operate very energy efficiently, since a laser beam only has to be turned on in each case when it is actually required for the projection.
However, it may be problematic in the case of such laser scanners that the image projected by laser scanners onto a surface generally has the so-called speckle effect. This speckle effect, which is also referred to as a light granulation effect, arises due to interference of the coherent laser light on unevenness of the surface and causes the image to appear grainy and partially slightly moving. The speckle effect is perceived to be unpleasant, which is presently detrimental to the market acceptance of the laser scanners.
Various methods are known for reducing the contrast of the speckle interference pattern. One of these methods provides the generation of a running, locally modulated interference strip system. Such an interference strip system is generated while using an ultrasonic cell in German Published Patent Appln. No. 197 10 660. The scanning laser beam is divided with the aid of the ultrasonic cell into various diffraction orders having different frequencies, which are subsequently superimposed again.